How do I know which faces correspond to which area of a UV image?
I try to answer in generic terms. Where texture mapping is concerned: there are no pixels!

The U and V coordinates are relative. They do not point to a pixel, they point to a position between 0 and 1. So if your texture has a resolution of 1024x1024 the coordinates 0.5, 0.5 would not point to a pixel, but to the interpolated color at the center. And this would be in that example right between the 4 pixels in the center of the texture resulting in a color value where all 4 pixels of the texture get mixed together in one.

If you UV map a 3D model, it is not guaranteed, that a texture coordinate lands right in the middle of exactly one pixel of your texture, it could land two thirds on one pixel, and one third on another, etc, etc.

When you export a uv map from blender so you can photoshop your texture over it, it is important to know that it does not matter where the edge is drawn. Just think of the edge as infinitly thin. There is no belonging to one specific face, if both share the edge.

However if you really want a "sharp" edge, one that is really separate from its neighbouring face, you can insert a seam in edit mode, unwrap the UVs again. Now there are two edges in the UV map. If you keep an eye on your vertex count you will see, that by making that seam, your vertex count goes up, since all vertices along the seam gets duplicated. They still occupy the same space and blender treats them as one, but the underlying renderer sees them separate.

I hope this answers you question.